High corrosion fatigue strength stainless steel

ABSTRACT

A high corrosion fatigue strength stainless steel is formed by combining less than 0.07 wt% carbon, less than 0.05 wt% nitrogen, from 0.1 to 2 wt% silicon, from 0.1 to 4 wt% manganese, from 10 to 15 wt% chromium, from 2 to 7 wt% nickel, from 0.1 to 3 wt% molybdenum, and a total of from 0.01 to 1 wt% of at least one element from the group of titanium, niobium and tantalum with the balance being iron, impurities and other incidental constituents.

nited States Patent Takamura et a1. Dec. 9, 1975 HIGH CORROSION FATIGUE STRENGTH 3,337,331 8/1967 Ljungberger 75/128 T STAINLESS STEEL 3,385,740 5/1968 Baggstrom 148/38 X 3,767,388 10/1973 Asakura 75/128 T X Inventors: Aklra Takamura, k h Kazutoshi 3,769,003 10/1973 Kenyon 75/128 w Shimogori; Toshiaki Yamagata, both 3,795,509 3/1974 Mimino et a1 75/128 T of Kobe; Masao Sato, Osaka; Koziro Kltanata Akashl of Japan Primary ExaminerL. Dewayne Rutledge [73] Assignee: Kobe Steel, Limited, K b Japan Assistant Examiner-Arthur J. Steiner Attorney, Agent, or FirmOblon, Fisher, Spivak, [22] Flled- May 1973 McClelland & Maier [21] Appl. No.: 365,714

[52] US. Cl 75/128 A; 75/128 C; 75/128 G; [57] ABSTRACT 2 75/128 T; 75/128 W; 148/38 A high corrosion fatigue strength stainless steel is [51] Int. Cl. C22C 38/44; C22C 38/48;

C22C 38/58 formed by comblnlng less than 0.07 wt% carbon, less than 0.05 wt% nitrogen, from 0.1 to 2 wt% silicon, [58] Field of Search f? from 0.1 to 4 wt% manganese, from 10 to 15 wt% chromium, from 2 to 7 Wt% nickel, from 0.1 to 3 wt% References Cited molybdenum, and a total of from 0.01 to 1 of at least one element from the group of t1tan1um, n1ob1um UNITED STATES PATENTS and tantalum with the balance being iron, impurities 2,496,248 1/1950 Jennings 75/ 128 W and other incidental constituents. 3,177,577 4/1965 Fujimura.... 3,301,668 1/1967 Cope 75/128 w 1 Claim, 5 Drawing Figures US. Patent Dec. 9, 1975 Sheet 1 of2 3,925,064

FIG. 1

I, L 7 Zmm L i i OSCILLATOR l 7 AMPLIFIER VIBRATOR COOLANT EXCITING COIL FIG. 5 NICKEL VIBRATUR TEST SPECIMEN [among :30

TEST LIQUID US. Patent Dec. 9, 1975 Sheet 2 of2 3,925,064

LIQUID VESSEL SUSPENDER L OUTLET FOR L COIROSIVE LIQUID SPECIMEN CORROSIVE LIQUID INLET FOR CORROSIVE LIQUID POLYTETRAFLUOROETHYLENE VESSEL HIGH CORROSION FATIGUE STRENGTH STAINLESS STEEL BACKGROUND OF THE INVENTION this invention relates to a stainless steel suited for casting and which is useful as a casting material for purposes such as ship engine propellers or water turbine runners.

2. Description of the Prior Art:

Recently, the production of large, high speed ships has imposed increasingly stringent requirements on the material design of ship propellers. For instance, such factors as corrosion resistance, resistance to cavitation damage and fatigue strength in the corrosive sea water environment, especially under sustained stress conditions, have become of substantial importance. Furthermore, from the repair or maintenance standpoint, very good weldability of propellers is required, particularly for water turbine runners. Previously, l3%-Cr martensitic stainless steel had been used for this application. However, whenever it became necessary to weld repair, austenitic welding electrodes had to be used because of the poor weldability of such steels. To some extent, the strength of the base metal was thereby sacrificed.

It has also been known to use manganese bronze or nickel-aluminum bronze alloys for sea water ship propellers in order to provide good castability, corrosion resistance and machineability. However, these types of alloys are no longer used because of the aforementioned severe material requirements imposed on ship propellers and water turbine runners.

A need, therefore, exists for a stainless steel which is suitable for use as a ship propeller, a hydrofoil or water turbine runner, and yet which has high corrosion fatigue strength and good weldability.

SUMMARY OF THE INVENTION Accordingly, one object of the present invention is to provide a stainless steel having high corrosion fatigue strength, high resistance to cavitation damage and good weldability as well as good mechanical properties, which is suitable for use as a casting material for ship propellers and water turbine runners.

Briefly, these objects and other objects of the invention as hereinafter will become readily apparent can be attained by a highly corrosion resistant and high fatigue strength stainless steel which containsless than 0.07 wt% carbon, less than 0.05 wt% nitrogen, from 0.1 to 2 wt% silicon, from 0.lto 4 wt% manganese, from l0 to wt% chromium, from 2 to 7 wt% nickel, from 0.1 to 3% molybdenum and a total of from 0.01 to 1 wt% of at least one of titanium, niobium and tantalum with the balance of the composition being iron together with impurities or incidental constituents.

BRIEF DESCRIPTION OF THE DRAWINGS A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 is a diagram of a welding test device;

FIG. 2 is an illustration of a steel specimen for use in a magnetostriction oscillation cavitation test device;

FIG. 3 is a diagram of a magnetostriction oscillation cavitation device;

FIG. 4 is a diagram of a corrosion fatigue test device showing the construction of a liquid vessel; and

FIG. 5 is an illustration of a steel specimen for use in a corrosion fatigue test device (mm).

DESCRIPTION OF THE PREFERRED EMBODIMENTS Progress toward the development of the stainless steel of this invention has been made possible by the following discoveries:

The addition of a certain amount of at least one of the elements Ti, Nb and Ta in the presence of M0 in a steel eliminates embrittlement of the steel which is caused by the presence of C and N compounds in the steel.

The corrosion fatigue strength of the steel is improved by minimizing the amount of C and N.

Appropriate concentrations of Cr and Ni also improve the corrosion fatigue strength and weldability of the steel.

Certain amounts of Mo also improve these characteristics.

Certain amounts of Si promote embrittlement of the steel.

The addition of manganese in certain concentrations not only provides considerable savings-by limiting the consumption of Ni as an alloy element, but also improves the toughness of the steel.

The stainless steels of the present invention are subjected to normalization at temperatures of from 900 to llOOC, or, after the normalization treatment, to tempering at temperatures of from 550 to 750C (stress relieving annealing). It is important, especially when the steel is cast that it be annealed one to three times at temperatures from 550 to 850C. The structure of a stainless steel treated in this manner consists essentially of a martensitic and austenitic structure. The concentration levels of the alloying elements and the heat treatments of the steel should be adjusted so that the retained austenitic level is in the range of from 1010 45%, and the ferrite level remains below 20%.

The present invention thus provides a high corrosion fatigue strength stainless steel especially suited for casting, which comprises less than 0.07wt% carbon, less than 0.05 wt% nitrogen, from 0.1 to 2 wt% silicon, from 0.1 to 4 wt% manganese, from 10 to 15 wt% chromium, from 2 to 7 wt% nickel, from 0.1 to 3 wt% molybdenum and a total of from 0.01 to 1 wt% of at least one of the elements titanium, niobium and tantalum with the balance being substantially iron and impurities.

The amount of carbon in the steel should be limited to less than 0.07 wt% because a carbon level exceeding the upper limit reduces the toughness of large products made from the steel, such as cast and slow cooled castings of stainless steels which may be prepared according to the procedure of the present invention. Furthermore, concentration levels of carbon above the limit indicated have adverse effects on the corrosion fatigue strength, corrosion resistance and weldability. These properties are optimized when the carbon level is maintained below 0.05%.

The amount of nitrogen in the steel should also be limited to less than 0.05% because it forms nitrides which cause embrittlement and thus lower the corrosion fatigue strength. The desired properties of the steel are optimized when the nitrogen level is below 0.03

The amount of Si in the steel can vary between 0.1

and 2%. Silicon is included in the steel melt in concentrations over 0.1% to act as a deoxidizer. However, if the concentration exceeds 2%, embrittlement of the steel is promoted.

Manganese should be present in the steel in amounts ranging from 0.1 to 4%. A minimum of about 0.1% of manganese is necessary as an austenizing element which improves the toughness of the steel and also serves as a deoxidizer. Manganese contents in excess of 4% are too high and do not provide any further improvement in the desired properties of the steel.

Chromium should be limited to within the range of from 10 to 15%. Chromium is an essential component in this type of stainless steel, since its presence greatly improves the corrosion resistance of the steel. To

achieve the objects of the invention, that is, to make a steel which possesses the desired degree of corrosion resistance in sea water and fresh water, it is necessary that the chromium be present in concentrations which exceed 10%. The corrosion resistance of the alloy increases with increasing chromium levels. Chromium contents in excess of 15%, are, however, detrimental as that high a percent increases the likelihood of formation of delta-ferrite steel which causes embrittlement and lowers the corrosion fatigue strength. The desired properites can be optimized when the chromium level is from 12 to 14%.

Nickel contents should be within the range of from 2 to 7%. The presence of nickel in the alloy serves as an outstanding austenite forming element. At least 2% nickel is necessary in order to improve the toughness and weldability of the steel. However, nickel contents in excess of 7% increase the proportions of retained austenitic character so that the required corrosion fatigue strength is not obtained. Nickel contents within the range defined above meet the requirement of a tent, which increases the corrosion fatigue strength of the steel without decreasing toughness, is interdependent with the chromium and molybdenum contents. For example, if from 12 to 14% Cr is used, from 4 to 6% nickel is a suitable amount.

Molybdenum is present in amounts ranging from 0.1 to 3%. Molybdenum contents of at least 0.1% are necessary to substantially increase the corrosion fatigue strength as well as to increase the resistance to corrosive environments such as sea water, in co-operation with carbide forming elements such as titanium, niobium and tantalum. If the molybdenum content exceeds 3%, however, no appreciable improvement is observed in these properties, but rather embrittlement is promoted and poor corrosion fatigue strength results.

At least one of the elements titanium, niobium and tantalum should be present in the steel in amounts so that the total content of these elements is within the range of from 0.01 to 1%. Titanium, niobium or tantalum in the presence of molybdenum supresses the precipitation of carbides or nitrides of chromium, and thus increases the corrosion fatigue strength. Further, they are effective in improving erosion and intergranular corrosion resistance. For these purposes, the titanium, niobium and/or tantalum content as single elements or in combination in the presence of molybdenum should be at least 0.01%. If the content of these elements is in excess of 1% the alloy will be susceptible to deleterious segregation, which lessens the toughness of the steel.

Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purpose of illustration only and are not intended to be limiting unless otherwise specified.

EXAMPLES Table I shows the compositions of various samples of steel and Table 11 gives the mechanical properties, resistance to cavitation damage and corrosion fatigue strength of these samples.

Samples 1 to 5 and 13 to 17 represent various stainless steel compositions according to the present invention. Samples 6 to 12' are various comparative examples whose compositions are outside the scope of the present invention. These samples were prepared by heating a 20 kg cast material to llC, then cooling the cast material at a rate of 15C/hr to room temperature. By this procedure were obtained large size castings having a heat history which corresponds to the history of a casting which has been cast, slowly cooled and then subjected to a heat treatment at a residual stress relieving annealing temperature.

As shown in Table II, the results obtained from the steels of this invention show considerable improvement 50 in properties over the steels of the comparative examples.

TABLE 1 CHEMICAL COMPOSITION OF STAINLESS STEEL SAMPLES (WT%) Sample No. C Si Mn P S Cr Ni Mo Ti Nb Ta N 1 Present 0.03 0.25 0.53 0.019 0.008 12.68 5.10 1.05 0.24 0.02 1 2 Invention 0.03 0.29 0.51 0.019 0.009 14.72 5.40 1.04 0.26 0.02 3 0.03 0.25 0.52 0.019 0.010 12.87 5.30 0.98 0.15 0.02

8 Comparative 0.08 0.42 0.79 0.022 0.007 12.45 5.95 1.47 0.02

9 Examples 0.03 0.23 0.49 0.017 0.009 12.91 5.11 0.25 0.02

13 Present 0.02 0.21 0.48 0.018 0.009 13.20 6.71 1.01 0.25 0.02

14 invention 0.03 0.28 0.53 0.019 0.010 12.98 3.33 0.97 0.27 0.02

TABLE II PROPERTIES OF SAMPLES g I 0.2p Field Tensile Elonga- Charpy impact Weight loss due Fatigue Strength Sample Strength Strength tion value(2mm V to cavitation in 3% NaCl No. (kglmm (kg/mm") C.)(kg.m/cm erosion (mg)* (kg/mm")** 1 Present 70.2 95.3 16 3.5 6.5 30

2 Invention 64.7 91.3 21 3.0 6.8 30

g 7 62.8 85.3 18 9.5 9.0 14 8 Comparative 75.3 95.7 17 8.5 6.2 13 a 9 j Examples 71.1 88.3 16 3.7 7.1 18 Y 10 g 60.2 90.0 8 0.8 7.5 17 11 f 61.5 90.2 16 2.0 8.2 12 59.2 91.4 18 1.1 7.8 18

13 Present 45.2 94.2 5.7 7.7 27 14 Invention 63.7 g 92.1 15 2.7 6.8 15 72.6 98.3 14 3.2 6.4 30 16 70.7 95.8 16 3.6 6.8 23 17 70.1 95.3 16 3.5 6.9 25

Remarks: (*)denotes the weight loss'ot' the steels due to cavitation erosion as determined by a vibratory test. (Test conditions: frequency 6.5 kc. amplitude. 6 microns, 3% NaCl, 25C, 3 hours). 7 (")denotes a rotation bending test (10 cycles).

Table III is a comparison of the properties of two FIGS. 2 to 5 are illustrations of other test devices stainless steels both having the composition of sample 1 used for testing the steels and samples of steel suitable wherein one steel was tested in an as cast condition and '30 for testing. the other was tested after the casted metal hadbeen TABLE Iv subjected to an annealing treatment. As is apparent from this table the stainless steels of the present inven- WELDING TEST RESULTS Sample No. Cracks after weldmg test t10n have impact values comparable to the comparative stainless steels and yet have a higher corrosion fatigue mm 1 5 Absent from 6 to 12 Present Strengthfrom l3 to l7 Absent TABLE III I HEAT HISTORY OF SAMPLES AND PROPERTIES THEREOF Sample No. Heat 0.2% Yield Tensile Elongation Charpy impact Weight loss Fatigue History Strength Strength value (2mm V due to cavi- Strength in (kg/mm) (kg/mm) notch 0C.) tation erosion* 3% NaCl (kg.m/cm*) (mg) (kglmm As cast 1 (Slow 50.2 82.2 5 1.0

Cooling) As cast :1050C. X 5 hours Furnace 53.5 86.4 22 4.6 6.9 30 Cooling :650C. x 5 hours and have the same meaning as described in Table II.

i Tables IV and V give the welding test results of the samples of the present invention and the comparative Table V glves the mechamcal propertles e the samples. The welding tests were conducted according fOSlOIl fatlglle g h o the We ding Specimensto the procedures of the so-called Y type weld Crack- TABLE V ing test which by nature is kind of a binding test. The particular test procedures themselves are of little signif- E 5g Q$;%%E }S S EEE ggg gfigg icance as this test is intended only for a comparison of 0.2% Yield Tensile Strength Elon ation Broken the samples. In order to obtain a better understandin g/ P of the test, an illustration of the test devise is shown in 8 FIG. 1. In this test, the root spacing is 2mm and the am- 18 Base meta bient temperature is room temperature. Charpy impact value (2mm v, 0C.) Bending Fatigue 7 TABLE V-continued MECHANICAL PROPERTIES & CORROSION TABLE v11 MECHANICAL PROPERTIES OF THE FORGED MATERIAL FATIGUE STRENGTH OF THE WELDING SPECIMENS 0.2% Tensile Elonga- Charpy impact Fatigue 0.2% Yield Tensile Strength Elon ation Broken Yield Strength tion value Strength in Strength (kg/mm) portion Strength (kg/mm) (2mmV,0C.) 3% NaCl (kg/mm) (kglmm (kg.m/cm (kg/mm) (kg.m/cm R3=02T Styreplgtglin 69.5 93.5 21 15.2 30

3 a Base Heat affected Deposit (kg/mm) metal zone 3 5 4 6 5 4 g k 0 10 The stainless steels according to the present invenme 5 3 tion can be readily made in a conventional electric arc furnace or in a high frequency induction furnace, and The test results shown in Table I to V show the data ne-cessltate 9 pamcular castmg i f The}, are obtained for the various corrosion fatigue strength, resulted especially for use as ma-tena S for Shlp Propel- Iers, hydrofoils and water turbine runners Wl'llCl'l reslstance to cavitation damage and weldability tests conl h h t h d ducted. The results show the superior Properties which fi il hlg corrosion fangue Strengt an goo we a 1 ity. lhzs c f tl i g r io i' if a ll ci yl s possess compared to Having now fully described this invention, it will be The stainless steels of the present invention can be apparent to one i onimary 5km m the art that a changes and modifications can be made thereto withused as forged, rolled and/or rod materials. The properh f h ties of these materials are the same as those of the out g g g frqm t e Spmt or Scope 0 t e mvemlon as set ort erem. casted metal. For example, Tables VI and VII show the chemical composition and the properties of a forged li gz g s s new and Intended to be covered ro m d (20 m diameter) accordmg to the present mven 1. A h1gh corrosion fatlgue strength stainless steel tained austenite.

TABLE VI CHEMICAL COMPOSITION OF THE FORGED MATERIAL C Si Mn P S Cr Ni Mo Nb N 0.03 0.3] 0.56 0.01 l 0.0l0 12.8l 4.98 0.97 0.27 0.02

Heat treatment: I050C X 3 hours air cooling followed by 650C X 5 hours air cooling. 

1. A HIGH CORROSION FATIGUE STRENGTH STAINLESS STEEL WHICH CONSISTS ESSENTIALLY OF LESS THAN 0.05 WT% CARBON, LESS THAN 0.03 WT% NITROGEN, FROM 0.1 TO 2 WT% SILICON, FROM 0.1 TO 4 WT% MANGANESE, FROM 12 TO 14 WT% CHROMIUM, FROM 4 TO 6 WT% NICKEL, FROM 0.1 TO 3 WT% MOLYBDENUM AND A TOTAL OF FROM 0.01 TO 1 WT% OF AT LEAST ONE ELEMENT SELECTED FROM THE GROUP CONSISTING OF TITANIUM, NIOBIUM AND TANTALUM WITH THE BALANCE BEING ESSENTIALLY IRON SUCH THAT SAID STEEL HAS FROM 10 - 45% RETAINED AUSTENITE. 